Hydrogen is playing an increasingly important role in clean combustion and zero-emission power generation. Fuel cells operating on pure hydrogen or hydrogen-rich gas (reformate) have the potential to revolutionize power generation for both stationary and transportation applications. Distribution of hydrogen to fuel-cell devices poses significant technical difficulties due to hydrogen's low energy density, poor distribution infrastructure, and high cost. Thus, a great and practical interest for fuel-cell power generation units is the conversion of liquid hydrocarbon fuels, such as gasoline, kerosene, jet fuel, diesel fuel and heating oil, to a gaseous stream rich in hydrogen. One common attribute of these fuels is that they all contain high concentrations of sulfur. Typical sulfur concentration in commercial grade gasoline, diesel and jet fuels currently range from 200 to 3000 parts per million by weight (ppmw).
Conversion of hydrocarbon fuels to hydrogen and carbon oxides is generally carried out in a reactor vessel via one of three processes: stream reforming, partial oxidation and autothermal reforming. To increase the process efficiency and reduce the operating costs, catalysts are normally used in all these reforming processes. Commercially available catalysts for these reforming processes include transition metals, such as nickel, and noble metals, such as platinum, supported on ceramic oxides. However, none of these catalysts can be used to directly reform high-sulfur fuels, such as diesel and gasoline, because the catalysts are extremely sensitive to poisoning by sulfur. Even a few ppmw sulfur may cause severe deactivation of these catalysts.
In addition to poisoning reforming catalysts, sulfur also has a detrimental effect on fuel cell performance and thus needs to be removed from the hydrogen or reformate. For the most-commonly used polymer electrolyte membrane (PEM) and solid oxide fuel cells (SOFC), the presence of less than one part per million by volume (ppmv) sulfur in the feed stream can result in an immediate drop in fuel-cell efficiency. Therefore, to protect reforming catalysts and fuel cells, sulfur must be removed from hydrocarbon fuels that are to be used as feeds for fuel-cell power generation systems.
Existing systems and apparatuses for converting hydrocarbon fuels to sulfur-free gas streams suitable for use in fuel cells are disclosed in U.S. Pat. No. 6,159,256; U.S. Pat No. 6,156,084; U.S. Pat. No. 6,210,821; U.S. Pat. No. 5,302,470; and U.S. Pat. No. 5,686,196. The first three of these patents disclose two complicated systems for reforming sulfur-containing hydrocarbon fuels. The first system is based upon stream reforming, and the second is based upon autothermal reforming. In both systems, a fuel desulfurizer is used to remove sulfur from raw fuels before reforming. The fuel desulfurization is based on the well-known mechanism of reactive adsorption of sulfur on transition metals, such a nickel (e.g., RSR′+Ni→NiS+R″H, where R, R′ and R″ are different hydrocarbon groups). Although the process is capable of treating low-sulfur fuels with very-high sulfur removal efficiencies (to <1 ppmw), the process suffers from an inability to treat high-sulfur fuels, such as diesel and jet fuels, which typically contain hundreds of ppmw of sulfur, because of the formation of dense NiS shells on the outer surfaces of Ni particles; the NiS shells result in very-low sulfur uptake. As pointed out by Anumakoda, et al., in U.S. Pat. No. 6,221,280, a Ni:S weight ratio of at least 100:1 is needed for near complete removal of residual thiophenes from diesel or jet fuels. Thus, fuel desulfurization by this approach is costly and demanding in terms of metal weight and reactor volume.
The fuel processing systems disclosed in U.S. Pat. No. 5,302,470 and U.S. Pat. No. 5,686,196 employed another approach for desulfurization of raw fuels prior to stream reforming to convert desulfurized fuel into hydrogen-rich streams. The desulfurization process disclosed in these two patents is based on the traditional hydrodesulfurization (HDS) process widely used in the petroleum refining industry. In the HDS process, a raw fuel is mixed with gaseous hydrogen and fed into a reactor containing CoMo/Al2O3 or NiMo/Al2O3 catalyst at high temperature and pressure. Sulfur compounds in the fuel react with hydrogen (hydrogenation) over the catalysts to form hydrogen sulfide via the following reaction: RSR′+2H2→RH+R′H+H2S. The H2S is then removed with a regenerable metal or metal oxide absorbent, such as ZnO or CuO, via the following sulfiding reaction: H2S+MO→MS+H2O. The two reactions can be carried out in a single reactor by mixing the hydrogenation catalysts and the absorbent materials together. It is well known that in order to reach high efficiencies in desulfurizing heavy hydrocarbon fuels, such as diesel and jet fuels, the HDS process has to be operated at very-high pressure and with large amounts of excess hydrogen. While the process may be effective for use in larger, stationary hydrocarbon reforming systems, it is not attractive for smaller, mobile fuel reforming applications because of its dependence on an external hydrogen feed, large system size, complexity, and high operating pressure.
U.S. Pat. No. 6,296,814; U.S. Pat. No. 6,120,923; U.S. Pat. No. 5,931,658; U.S. Pat. No. 5,516,344; U.S. Pat. No. 6,254,839; U.S. Pat. No. 6,126,908; and U.S. Pat. No. 6,232,005 disclose a number of integrated apparatuses for reforming light hydrocarbon fuels. None of these patented designs contained a fuel desulfurizer, and thus these apparatuses are not employed to convert high-sulfur fuels into sulfur-free hydrogen or reformate for clean combustion or fuel cell power generation.
Consequently, there is a need for compact and efficient fuel processors for converting sulfur-containing hydrocarbon fuels into sulfur-free and hydrogen-rich gaseous streams.